1. The Field of the Disclosure.
The present disclosure relates generally to airfoils or watercraft structures, and more particularly, but not necessarily entirely, to airfoils utilizing washout to minimize induced drag.
2. Description of Related Art
Induced drag is caused by the generation of lift by a wing and is parallel to the relative wind into which the wing is flying. When a wing flies at the zero lift angle of attack there is no lift and therefore no induced drag. Conversely, when the angle of attack increases the wing produces more lift, therefore there is more induced drag. The magnitude of the induced drag depends on (1) the amount of lift being generated by the wing; and (2) on the shape and size of the wing, also known as wing planform. As might be expected, induced drag is undesirable while flying in that it results in diminished fuel economy as well as decreased airspeed. Induced drag also contributes to the stall characteristics of a given wing.
The prior art teaches various features that may be incorporated into a wing in order to reduce induced drag at high angles of attack. One of the more well known ways to reduce induced drag is to increase the wingspan. For example, this would include aircraft such as gliders, as well as high altitude spy planes such as the U2. It also includes to a lesser degree modern jet airliners. However, as the span is increased, the wing structural weight also increases and at some point the weight increase offsets the induced drag savings.
Another previously known method for reducing induced drag is to employ end plates onto the tips of the wings. The end plates served to block some of the vortices causing reduced drag. However, end plates are not employed widely due to their relative inefficiencies. Still another method for reducing drag is using winglets. Unlike the other methods mentioned above, the winglet does not strive to reduce induced drag so much as it uses it to create an offsetting thrust. However, winglets cannot be used on all planes due to performance considerations which are not discussed here. Other known attempts to reduce induced drag include wings with slotted edges and wings with fanned partial wings.
Tapered wings are also commonly used as a means for reducing induced drag. It can be shown that tapered wings with the right amount of taper have a lower reduced drag than an untapered wing. However, this reduction comes at a price. A tapered wing tends to stall first at in the region near the wingtips. This wingtip stall can lead to poor handling characteristics during stall recovery. Thus, tapered wings have commonly been used as a compromise solution.
Around the 1920s it was found that a flat elliptical shaped wing gave a uniform air deflection along the entire span, which minimized the induced drag. Elliptical shaped wings were used on the British SuperMarine Spitfire, a popular WWII fighter, to reduce induced drag. In fact, it can be shown that an elliptical wing produces the minimum possible induced drag for all angles of attack. Unfortunately, there are several problems with elliptical wings. First, elliptical shaped wings are cost prohibitive. While this barrier is less important today than it once was, provided that the designer is willing to use modern composite materials. However, making an elliptical shape out of aluminum is quite difficult and therefore expensive. Next, elliptical wings have undesirable stall characteristics. It is much safer to design an airplane so that the wing stalls first at the root, leaving the outer portion of the wing, (where the ailerons are) still flying. An elliptical wing however, will tend to stall uniformly all along the span creating a potentially dangerous situation for the pilot. Finally, other factors dictate a wings ideal shape more than the desire to reduce induced drag. The tapered wing, for instance, is lighter and easier to build, factors which outweigh the advantages of an elliptical wing's ability to reduce induced drag.
Another popular method of reducing induced drag is to design a wing with washout, also referred to herein as twist or wing twist. Washout may be applied to wings so that the outboard section of the wing does not stall first. When an aircraft may be increasing its angle of attack, i.e. increasing the lift of the wing, the airflow over the wing eventually reaches a point where it becomes turbulent, causing a loss in lift. By twisting the front outboard portion of the wing down, the induced drag in that area may be decreased and the stall may be delayed in that area. By maintaining lift on the outboard portion of the wing, the pilot may be still able to maintain roll control of the aircraft in the event of a stall on other portions of the wing.
Conventionally, washout may be incorporated into a wing using geometric twist and aerodynamic twist. The use of washout in the prior art, however, may be characterized by two major shortcomings. First, since the amount of twist may be integrated into a wing at the time of construction, usually for a design lift coefficient, the twist in a wing may only be optimized, if at all, for one portion of the expected flight envelope. Second, washout comes at a price. A wing with washout experiences a decrease in lift performance due to the reduction in the angle of attack.
The prior art is thus characterized by several disadvantages that are addressed by the present disclosure. The present disclosure minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.
The features and advantages of the disclosure will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the disclosure without undue experimentation. The features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.